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Galois Representations with Non-Surjective Traces

Published online by Cambridge University Press:  20 November 2018

Chantal David
Affiliation:
Concordia University, Department of Mathematics, 1455 de Maisonneuve Blvd. West, Montréal, Quebec, H3G 1M8 email: chantal@cicma.concordia.ca
Hershy Kisilevsky
Affiliation:
Concordia University, Department of Mathematics, 1455 de Maisonneuve Blvd. West, Montréal, Quebec, H3G 1M8 email: kisilev@cicma.concordia.ca
Francesco Pappalardi
Affiliation:
Università degli Studi di Roma Tre, Dipartimento di Matematica, Via Corrado Segre, 4, 00146 Roma, Italy email: pappa@mat.uniroma3.it
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Abstract

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Let $E$ be an elliptic curve over $\mathbb{Q}$ , and let $r$ be an integer. According to the Lang-Trotter conjecture, the number of primes $p$ such that ${{a}_{p}}\left( E \right)=r$ is either finite, or is asymptotic to ${{C}_{E,r}}\sqrt{x}/\log x$ where ${{C}_{E,r}}$ is a non-zero constant. A typical example of the former is the case of rational $\ell $-torsion, where ${{a}_{p}}\left( E \right)=r$ is impossible if $r\equiv 1\,\left( \bmod \,\ell \right)$. We prove in this paper that, when $E$ has a rational $\ell $-isogeny and $\ell \ne 11$, the number of primes $p$ such that ${{a}_{p}}\left( E \right)\equiv r\,\left( \bmod \,\ell \right)$ is finite (for some $r$ modulo $\ell $) if and only if $E$ has rational $\ell $-torsion over the cyclotomic field $\mathbb{Q}\left( {{\zeta }_{\ell }} \right)$ . The case $\ell =11$ is special, and is also treated in the paper. We also classify all those occurences.

Keywords

Type
Research Article
Copyright
Copyright © Canadian Mathematical Society 1999

References

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